What is the difference between these? 2

[pmarc]

The question is simple:
What is the difference between two cases shown on attached picture in terms of process of establishing datum axis for total runout measurement? Is there any or not really?

http://files.engineering.com/getfil...-43cc-83c1-857a300493c9&file=cont_feature.pdf

I am not asking which of the approaches is better or is there any other option, I would just like to know if you see any difference and if yes, how this can be explained. Thanks a lot.

 

[pmarc]

Frank,
You are right, it does not contain. It is a second sketch from your 100+ thread posted by me on 6 Feb 12 6:25 and a modification of my first one.

The thing is that Dean, Evan, J-P clearly agreed that for all 3 cases there was no difference on how relationship between datum feature simulators A and B look like. I have been saying it is not the case and since I have seen this one as an analogy to the original, I am pretty sure that if we find an agreement on this, we will also find it on the original. Unfortunately it seems we are far far away from a consensus.

 

[ptruitt]

The tolerance analysis software package that I am most familiar with refuses to analyze point cloud data using A-B datum references. Perhaps they are waiting for folks to agree or another Y14.5 iteration to clear things up.

Peter Truitt
Minnesota

 

[pmarc]

Dean,
Let's leave it, okay? Actually I really got tired fighting this endless battle. You have your interpretation, I have mine. We can even do voting, if you like - and probably most of the folks will agree with you. But this will not change my point of view in any way. Sorry.

 

[fsincox]

Pmarc,
I do not expect to change your mind, either, I just want to post my responce to your OP.
You really pushed me into many other areas I have been thinking about for a while. First I want to restate my belief that the ASME standard is a good work, I spend so much time here tearing it down that I suspect people might wonder. I doubt I could do any better, but, it is clearly not perfect.
Your objections to my example seem to be based on the concept of how do you gage it. “What is my tolerance to make a gage for it?”
I have been suspecting that the discussion of the ASME standard has become too reliant on gaging to express principles. We all know parts are often not checked that way and some have even stated, here, that the use of gages is declining. Small machine shops, like mine, rarely rely on gaging and the idea that you have to gage to check part tolerances does not come from the text of the standard, people only infer this from the “pictures in the book”.
No one can be expected to stop production until they have a gage to check a part. It is ridiculous to say that all parts must be gaged . Gages do have their beneficial qualities like they actually check all points at once so it is definitely a useful tool to have in your box, but, it ain’t the only game in town, nor should it be. Most companies, just by judging by the voting of their dollars, don’t seem to be willing to invest them there.
I also think the over reliance on gaging in the standard starts to make it irrelevant to the majority of the people who actually use it, while, it certainly makes it easier for those who want to teach it. The ISO must feel that way, too, as they seem to be heading down the path of making it more relevant to CMM measurement. This is also almost certainly why the ASME is interested in the mathematical definition of the standard so it is relevant outside the gaging world.
The standard, as I was taught, basically has always thrown away the tolerance you are looking for. How perfect is a flat surface plate, how coaxial are the 2 “A-B” chucks, what is the tolerance of a basic dimension on a gage? How perfectly perpendicular are the three mutually perpendicular planes of measurement? The answers have always been something like: “we do the best we can do to simulate this perfection, do we achieve it, no? Do you achieve it, no? Is yours better that mine, maybe? The concept of perfection is implied all over the standard.
Your OP example relies on our ability to establish the primary datum, mine is an establishment of a secondary datum. To me there is a difference. The primary datum establishes the orientation of the axes of measurement of the part to the whole world. The secondary and tertiary datums refine this, but, do not stop us from getting started.
I can find the axis of my shaft. Yours we can put in chucks/collets no matter how eccentric they are. Do I know how perfectly coaxial those chucks/collets will be, no but, we treat them as perfect. I am just not sure both of your examples parts are “identical”. I did not say, either of the examples can’t be done.
When those of us who do not use gages check our parts we check them to the best of our ability: establish the center of the shaft, roll the keyway to center on to center as perfect as we can, how perfect is it? There is the tolerance you are looking for. We know the keyway is to be located on zero, the standard supports it in the shown examples, if not directly in the text. The fact you don’t have a tolerance to build your gage has never been part of this standard, you would have to build it the best you can like all the other measurements made to the standard.
When Mr. “doesn’t recommend bothering to tolerance the orientation of short secondary datum pilot diameters” builds his gage it doesn’t bother him either it has to be perfect, how perfect? It was not an issue to him, either. Do you object to his use too? I do feel it is very bad practice for him to teach this and I really did not recommend using the method I discussed either. I just said it was not illegal.
My point is your issue is really an ASME “rules of the game” issue not my “rules of the game” issue. The issue of perfection permeates the ASME standard to say it can’t be done is arguing with them not me. Remember, I am an Independency supporter for just this reason. Zero position tolerances at MMC, why are they not used more often and resisted buy the shop, in horror? Why do they ignore the envelope principle in the shop making ASME’s insistence on it a paradox for real day to day engineers (as opposed to people who teach)? The ISO do not really by it completely, either, they allow it to be used because the biggest gorilla in the room insists on clinging to it.
In summary, 14.5 is not a gaging standard and it makes no provision for it, which is why you have a standard specifically for that. The reliance of the standard on the concept of perfection does threaten to make it irrelevant but those are their rules of the game not mine. You can’t let some play the game one way and not let all do it and be taken serious with me. The part you show is not the same condition as mine and yet it to could be checked in theory, I just don’t know if it the same (the actual question). Isn’t the fact that they had to add a statement like 4.5.2 to the 2009 standard, proof enough that they had to finally admit what was obvious to some of us long ago, already?

Frank

 

[MechNorth]

There is a difference between the two OP cases, and it is based on the simulators. In either case, there is an implied basic-zero location establishing the coaxiality intent between the two datum features. In case 1, however, a single simulator is intended to engage both features simultaneously. In the second case, two separate coaxial simulators are intended to engage the two features simultaneously at their optimal AME.
That being said, if the two datum features in #2 are produced initially as a single feature before the center portion is relieved, then you would have the same result as #1.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[axym]

pmarc,

I think you've finally convinced me that you are right on this one.

I looked up the text of Section 1.4 (k) in the Fundamental Rules. I had not done this before today, despite the fact that you referred to it in the other thread. I thought I remembered what it said, but it looks I did not.

Here is the text, for any others like myself who did not bother to actually look it up:

"A zero basic dimension applies where axes, planes or surfaces that are shown congruent on a drawing and geometric controls establish the relationship between the features."

I did not realize that this section explicitly states that a geometric control was required in order for the zero basic dimension to apply. 1.4 (k) actually makes a more compelling case than 4.9 - I wish that you had mentioned it again in this thread to hammer it through our thick skulls. We (at least Dean and I) fixated on 4.5.2.

In your Case #2, the axes of A and B are shown congruent but do not have geometric controls establishing the relationship between them. So by 1.4 (k), the zero basic dimension does not apply. Even though 4.5.2 requires the simulators to have basic location relative to each other, the basic location is not defined. I see your logic now, I apologize for not following it all the way through earlier.

It would appear that 4.5.2 and 1.4 (k) conflict with each other or are unclear at best.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

Thank you Evan for being patient enough to analyze it deeper - not only through the perspective of 4.5.2, which in this case seems to create confusion rather than offer any help.

 

[fsincox]

I saw that just recently too. I agree that seems to cover it now, to make it illegal.
Frank

 

[greenimi]

Should I say something, should I not? Should I stir the pot?
Let do it:
Will this new development of this thread "What is the difference between these?" will affect somehow the outcome for the "back-door location" thread "Spinoff2: Using orientation to control position"
(datum feature simulators as a decision turning point)

 

[fsincox]

You are certainly welcome to as you were interested in expanding on it, this would directly affect your situation as it would make it a no-go, particularly if you wanted to reference the current ASME standard.
Frank

 

[dtmbiz]

1. For the CF version the features must be treated as a single feature and fit within “a” perfect envelope at MMC. This the requirement would dictate a single datum feature simulator. The features couldn’t be offset by 1mm and meet the limits of size (0.4mm in this case).

2. The A-B version would require 2 coaxial (4.12.2 fig 4-25 indicates A-B are coaxial) datum feature simulators that would allow the axis to be offset 1 mm. This would set up an angle difference between the datum feature simulators and the cylinder with the runout control . The distance between the features would impact that angle.

"4.12.2 Figures 4-24 and 4-25 are examples of a single datum
axis established from the axes of the datum feature simulators
that constrain the two coaxial diameters simultaneously."

3. Additionally the depending on the length of the part would potentially incur more cost for a long enough simulator.
I would conclude the 2 versions could yield different inspection results and inspection cost could be a factor.

Note:
ASME Y14.100
Shall is mandatory.
Will is a declaration of purpose.
Should or may are non -mandatory

 

[axym]

This "new" development (it's new for some of us, anyway) does affect the back-door location thread to a certain degree. The secondary datum feature would need to be located to the primary datum feature with a Position or Profile tolerance, in order for the two simulators to be perfectly in line.

I think that the back-door location effect would still exist, however. If the Position tolerance on the secondary datum feature was large and the Parallelism tolerances on the slot surfaces were small, then there would be back-door location refinement.

Does that make sense?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[GDTcoach]

Back to the original question. Let's put a position tolerance of 0 at MMC on the two features serving as datum features A and B. Then it has the same meaning as CF.
As Evan said, in the first case with CF, one simulator would close in on the two features. The result could be that the part could rock due to different size features.
In the second case, in my opinion, two coaxial simulators close in on the features. This will make for a more stable set up and probably reflect how the part functions.
I have uploaded an edited video from our Premium Site. The part is the Output Shaft for the Air Clutch/Brake I manufacture under the name of Inter-Mec. The shaft rides in two bearings. Together these bearings establish an axis of rotation. I want it inspected the same way it functions. The last part of the video shows how John Rivers uses a 4 jaw chuck and indicator to align the part to the axis of the lathe using the two datum features.
This is what I always thought a A-B datum feature reference meant. The Y14.5 committee isn't so convinced and will continue to debate this for a while I am certain. So, to clarify, I probably should add a note which is always an option. At this point my supplier has tribal knowledge and knows what to do.

 

[fsincox]

Don,
I think you are the guy I am always picking on! Nothing personal, I love most of your work, but I think you are the guy I was refering above, I notice I can no long find the example I thought I saw on your site, either, so it is either pussy foot around the issue or come right out and say it. Welcome here, by the way!
pmarc, I hope your last name is not "Foster".
Frank

 

[GDTcoach]

Hi fsincox, If you are referring to the perpendicularity comment, I have dealt with enough PPAP submissions where perpendicualrity was applied to a hole in sheet metal so no data could be gathered. At some point a little common sense has to kick in.

 

[fsincox]

Don,
Yes, PPAP?
What is the delta modifier, by the way?
It has been my impression from reading the standards, and the way I was first taught it, a philosophy of it being the job of a drawing to define the final acceptable part requirements, not to determine inspection methods and processes? And if the part is really not rigid (as in the case of sheet metal) why is it not their common sense to say well he can bend it there at final assembly. “Common sense” is a popular word in the political world, because it is hard to argue against. Generally it seems to be more of a “think like I do” kind of thing than a real arbitrary measure, Just like “standard” to some is more “done my way” than the way the rest of the world does it. Maybe I am falling into that same trap.
I see datums applied to parts without any additional definition all the time. From my experience all people need is another excuse not to use GD&T.
You, your self, find it necessary to remind people of the envelope principle and the simultaneous requirement; it is mainly because these fundamental principles are ignored in the shop not because people forget. Using a standard with rules that are not followed ends up not being a very useful tool kit at all. If people like yourself will not follow the rules why would anyone else?
Frank

PS I was not aware there was still an issue with A-B datum systems?

 

[DeanD3W]

OK, I was missing 1.4(k) in pmarc's assertions (which is a new addition to 1.4 that was made in 2009)... I do see the conflict between 1.4(k) and 4.5.2.

What if Parallelism applied the way Frank described was considered the geometric tolerance that establishes a relationship among the features? If the zero basic is present then the Parallelism does have this effect, since the simulator for datum feature B is then located wrt the simulator for datum feature A (again, not that I would rely on the location control provided by the Parallelism - It would be better just to be more clear and use a Position for this case). Of course this is a bit of a "Chicken and egg" assertion, since the Parallelism only provides the location control if the zero basic is present for the B simulator, and the zero basic is present per 1.4(k) only if the Parallelism provides this control... Fun .

I think the orientation refinement that Evan speaks of above could take over location control if the Position tolerance is large enough. I also think that it could be argued that the Parallelism itself meets the requirement of a geometric tolerance that establishes a relationship...

Either way, an interesting topic to discuss. This has made me examine 4.5.2, 4.9, and now 1.4(k) more closely. I think the location controlling effect of the Parallelism applied in the specific way that Frank described still should be considered to be true, even though I would never recommend relying on this effect.

Dean

http://www.d3w-engineering.com

 

[Belanger]

I don't think 1.4 is relevant to this issue of parallelism and back-door location.
I must admit that I tuned out of this discussion for a little while, but the specific point about 1.4(k) vs. 4.5.2 doesn't seem blatantly problematic. (Clarification in the next version of the standard would be good, of course.)

Here's what 1.4(k) says: "A zero basic dimension applies where axes, center planes, or surfaces are shown coincident on a drawing, and geometric tolerances establish the relationship among the features."

I think the key is that the last part says "...establish the relationship..." not "...establish any relationship..."

So to me having a parallelism callout on there doesn't automatically mean that the location is a basic dim of zero -- because "the relationship" is purely orientation.
However, 4.5.2(c) does say that the datum feature simulators must be basically located, and this is where the parallelism becomes a "back-door" location control. But it's because of 4.5.2(c), not 1.4(k).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[fsincox]

I agree, This discussion has provided me a good opportunity to reread those sections in the new standard and I have been trying to find similar statements in the 1982 version.
Please give me your impression on this statement:
I thought there was some agreement in the past that things are not retroactive? Say a FOS in 82 it does not say “directly opposed surfaces”, just “plane parallel surfaces”. My take in our discussion here was that the newer tighter definition would not be applicable if it did not also reference a newer standard, did I just want to believe that so hard I made it true in my mind, guys? Does anyone have a relevant experience in this area?
Frank

 

[MechNorth]

Standards are tricky things; technically, the only standard is a mandated standard (such as electrical codes, etc.) which is federally required. Groups like ASME work with voluntary standards, which allow users to adopt or not, as they see fit. Mandated standards automatically supersede earlier standards. Voluntary standards do not supersede earlier standards.

Newer versions of ASME standards do not automatically supersede earlier versions.

Jim Sykes, P.Eng, GDTP-S
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